In North American railroad history one of the more common types of freight car rolling stock has been the gondola car. Gondola cars have been used to transport many different kinds of freight, from bulk commodities to scrap steel. Traditionally, gondola cars have tended to have two relatively deep side beams. Typically, the side beams, the floor, and the end walls of the body of a gondola car define an open topped container, or receptacle, into which lading may be placed. Gondola cars may sometimes have a center sill of relatively modest size. The side beams may often be the dominant vertical load bearing members, and may tend, at their ends, to be mated to a laterally extending main bolster and shear plate. The side beams themselves have tended to be deep beams having a top chord, a side sill, and a vertical web extending between the top chord and side sill.
The top chord is, typically, a continuous chord member running substantially the full length of the car. The top chord defines the upper edge or upper margin of the side beam of the car. It performs the function of the upper flange of the side beam. Most typically the top chord may be a hollow section. While top chords in the form of I-beams and C-channels can, and have, been used, top chords are frequently formed of closed hollow sections, such as rectangular (or square) steel tubes. Most often, vertical lading in the gondola car may tend to cause the top chord to be placed in compression.
Similarly, a side sill may be, or may include, a bottom chord of the deep side beam. That is, the side sill may include a lengthwise running member that defines the lower bounding member of the side beam of the car. The lengthwise running member may run substantially the entire length of the side beam, and may function to define the lower flange of the side beam. That lengthwise member is sometimes called a side sill, and sometimes called a bottom chord, but in either case may tend to function as the lower flange of the side beam. The side sill terminology may be more commonly used where the longitudinally extending member links the ends of cross-bearers and cross-ties at the edge of a deck or floor. In use, under vertical load the bottom chord or side sill, as it may be called, is most typically in tension. A side sill or bottom chord member may typically tend to be of quite substantial cross-sectional area. It may have a cross-sectional area of a comparable order of magnitude to that of the top chord. It may not necessarily be of closed hollow section, but may, for example, have the form of a large angle iron. Under vertical loading, the top chord and bottom chord may tend to work in opposition to carry bending moments from the center of the car to the end sections, with the vertical side sheets of the car carrying shear between the top chord and the bottom chord.
There has long been a desire in the railroad freight carrying industry generally to reduce the weight of freight cars, and to increase the ratio of allowable lading weight to car weight. All other factors being equal, a lighter freight car may tend to permit a greater amount of lading to be carried without exceeding a maximum gross weight on rail, and may tend to reduce the amount of fuel consumed while backhauling empty cars. In as much as bottom chords and side sills may tend to be quite heavy, a very substantial reduction in the size and weight of a side sill, or the substantially total elimination of a side sill may therefore hold out the prospect of a significant reduction in weight. There may also be significant gains in simplicity of manufacture.
It may also be desirable, from time to time, to be able to clean out a gondola car, as when it may be desired to carry a different type of lading.